This invention relates to a sensor system, particularly to a system for determining the pH of a liquid medium and a system for determining the concentration of carbon dioxide in a liquid medium. The invention is also concerned with a method for measuring the concentration of carbon dioxide in a medium.
The measurement of desired parameters in various media, particularly in biological systems, is frequently required. For example, the measurement in blood of pH levels and concentration of gases, particularly oxygen and carbon dioxide, is important during surgical procedures, post-operatively, and during hospitalization under intensive care and many devices for the measurement of said physiological parameters have been suggested in the art.
U.S. Pat. No. 4,003,707, Lubbers et al, and its reissue patent Re 31879 a method and an arrangement for measuring the concentration of gases and the pH value of a sample, e.g. blood, involving the use of a fluorescent indicator at the end of a light-conducting cable which is sealingly covered by or embedded in a selectively permeable diffusion membrane. The radiation transmitted to and emitted from the indicator must be passed through various filtering elements and light elements, including reflectors, beam splitters and amplifiers before any meaningful measurements can be made.
U.S Pat. No. 4,041,932, Fostick, discloses a method whereby blood constituents are monitored by measuring the concentration of gases or fluids collected in an enclosed chamber sealingly attached to a skin "window" formed by the stratum corneum over a small area of the patient's skin. The measurements in the enclosed chamber are made, inter alia, by determining the difference in intensity of light emitted from a fluorescent indicator.
U.S. Pat. Nos. 4,200,110 and 4,476,870, Peterson et al, disclose the use of a pH sensitive indicator in conjunction with a fiber optic pH probe. In each of these patents the dye indicator is enclosed within a selectively permeable membrane envelope.
U.S. Pat. No 4,548,907, Seitz et al, discloses a fluorescent-based optical sensor comprising a membrane immobilized fluorophor secured to one end of a bifurcated fiber optic channel for exposure to the sample to be analyzed.
Many fluorescent indicators sensitive to pH, and thereby useful for pCO.sub.2 measurements, are known in the art. Examples of useful fluorescent indicators are disclosed in the above patents and also in "Practical Fluorescence" by George E. Guilbault, (1973) pages 599-600.
Sensor devices using fluorescent indicators may be used for in vitro or in vivo determinations of components in physiological media. For in vitro determinations the size of the device is normally of no consequence, but for in vivo use, the size of the sensor may be extremely critical and there is an increasing need in the art to miniaturize sensor devices, particularly catheter-type devices, for the in vivo determination of components in physiological media, e.g. blood. However, diminution in size of the components of such devices, particularly in the size of the sensor itself, decreases the strength of the signal emitted by the indicator and consequently presents problems in the detection and measurement of said signal. These problems are aggravated when the detector system requires a multiplicity of components, such as filters, beamsplitters and reflectors to isolate and measure the emitted energy. Each of said components reduces the emitted signal strength resulting in a sequential loss of measurable signal. Consequently, the more components present in the system, the weaker the final signal strength.
The problems associated with miniaturization of sensor devices are substantially solved by a device involving a radiation-transmissible junction of optical fibers encased in an opaque radiation reflective jacket as described and claimed in commonly assigned patent application Ser. No. 874,927 (U.S. Pat. No. 4,927,222).
With the aid of said device the emission signal from radiation-sensitive indicators, particularly fluorescent indicators of the type disclosed in the prior art references discussed above, may be received substantially unattenuated in a suitable detector without the necessity of filters, beam splitters, reflectors or other light elements used in the prior art.
Another approach for obtaining a meaningful measurement is to use the ratio of two signals which provides a signal with greater resolution than that obtainable from prior art systems based upon a single signal. Zhang ZHUJUN et al in Analytica Chimica Acta 160 (1984) 47-55 and 305-309 disclose that the fluorescent compound 8-hydroxy-1,3,6-pyrenetrisulfonic acid, referred to herein as HPTA, fluoresces when excited by excitation radiation having wavelengths of 470 and 405 nm and the fluorescence emission is sensitive to changes in pH in the physiological range of 6 to 9.
In contrast to the system disclosed by Zhujun et al, which uses two excitation radiations to produce fluorescence, surprisingly, it has now been found that highly accurate, stable determination of pH can be obtained from a single external source of excitation radiation which is used to excite a first fluorescent indicator which in turn emits fluorescent radiation to excite fluorescence emission in a second fluorescent indicator, e.g. HPTA; said first indicator being insensitive to pH.
According to the present invention, a new improved system is obtained by the use of two fluorescent indicators acting in concert or by the use of a single fluorescence indicator which emits fluorescent signals of different wavelengths in different carriers which signals have intensities proportional to the parameter under investigation. Under this approach the parameter being measured is determined by the ratio of two diverging signals which provides greater resolution and a highly accurate, stable determination.
The term "stable" as used herein is intended to mean the stability of the determination with respect to all factors which might influence the measurement other than the parameter being measured. Thus the determination is not affected by, for example, changes in the strength of the excitation radiation, fluctuations in light or temperature or minor equipment defects. Since the quantity being measured is a ratio between two given intensities and this ratio remains constant when the value being measured is constant, irrespective of the actual size of the individual intensities, the resultant determination is necessarily stable.
When the excitation radiation used to actuate the system according this invention is introduced through a device as claimed in U.S. Pat. application Ser. No. 874,927 even greater signal strength and resolution may be obtained.